FIG. 1 depicts a schematic diagram of an exemplary wireless local-area network (LAN) 100 in the prior art comprising access point 101, stations 102-1 through 102-N, wherein N is a positive integer, and hosts 103-1 through 103-N, interconnected as shown. Each station 102-i, wherein i is a member of the set {1, 2, . . . N}, enables respective host 103-i (a device such as a notebook computer, personal digital assistant [PDA], tablet PC, etc.) to communicate wirelessly with other hosts in local-area network 100 via access point 101.
Access point 101 and stations 102-1 through 102-N transmit blocks of data called frames. A frame typically comprises a data portion, referred to as a payload, and a control portion, referred to as a header. Frames transmitted from a station 102-i to access point 101 are referred to as uplink frames, and frames transmitted from access point 101 to a station 102-i are referred to as downlink frames. A series of frames transmitted from a station 102-i to access point 101 is referred to as an uplink traffic stream, or simply an uplink stream, and a series of frames transmitted from access point 101 to a station 102-i is referred to as a downlink traffic stream, or simply a downlink stream.
Access point 101 and stations 102-1 through 102-N transmit frames over a shared-communications channel such that if two or more stations (or an access point and a station) transmit frames simultaneously, then one or more of the frames can become corrupted (resulting in a collision). Consequently, local-area networks typically employ protocols for ensuring that a station or access point can gain exclusive access to the shared-communications channel for an interval of time in order to transmit one or more frames.
Such protocols can be classified into two types: contention-based protocols, and contention-free protocols. In a contention-based protocol, stations 102-1 through 102-N and access point 101 compete to gain exclusive access to the shared-communications channel, just as, for example, several children might fight to grab a telephone to make a call.
In a contention-free protocol, in contrast, a coordinator (e.g., access point 101, etc.) grants access to the shared-communications channel to one station at a time. An analogy for contention-free protocols is a parent (i.e., the coordinator) granting each of several children a limited amount of time on the telephone to talk, one at a time. One technique in which a coordinator can grant access to the shared-communications channel is polling. In protocols that employ polling, a station that wishes to be polled submits a polling request (also referred to as a reservation request) to the coordinator. The coordinator subsequently sends a poll to the requesting station, granting that station exclusive access to the shared-communications channel for an interval of time. Since the coordinator polls only one station at a time, and stations transmit only in response to a poll from the coordinator, polling-based protocols can provide contention-free access to the shared-communications channel.
In some local-area networks where access point 101 acts as the coordinator, such as those based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard, access point 101 combines, when possible, a payload and a poll into a single downlink frame. For the purposes of this specification, such a frame is referred to as a downlink data/poll frame.